A novel route to the fluorinated diimines: Carbon monoxide-promoted reductive homocoupling of fluorinated imidoyl iodides in the presence of a palladium catalyst

Article abstract of DOI:10.1039/b303040g

A new catalytic access to the fluorinated diimines which involves palladium(0)-catalyzed reductive dimerization of the imidoyl iodides is presented.

Full text of DOI:10.1039/b303040g

A novel route to the fluorinated diimines: carbon monoxide-promoted
reductive homocoupling of fluorinated imidoyl iodides in the presence of
a palladium catalyst†
Hideki Amii, Mitsuhiro Kohda, Motoharu Seo and Kenji Uneyama*
Department of Applied Chemistry, Faculty of Engineering, Okayama University, Okayama 700-8530, Japan.
E-mail: uneyamak@cc.okayama-u.ac.jp; Fax: +81-86-251-8021; Tel: +81-86-251-8075
Received (in Cambridge, UK) 19th March 2003, Accepted 28th May 2003
First published as an Advance Article on the web 18th June 2003
A new catalytic access to the fluorinated diimines which
involves palladium(0)-catalyzed reductive dimerization of
the imidoyl iodides is presented.
reductant for Pd-catalyzed homocoupling of fluorinated imi-
doyl iodides 3.8
When a mixture of 3a and K2CO3 in DMF–toluene was
heated at 70 °C for 24 h in the presence of a catalytic amount of
a Pd(0) complex under argon atmosphere essentially no reaction
was observed, resulting in recovery of the starting material 3a
(entry 1 in Table 1). A dramatic change was observed when the
reaction was conducted under CO (1 atm) to provide the
homocoupling product 2a in 64% yield (entry 2).‡ Carbon
monoxide would play an important role for regeneration of
catalytically active Pd(0) species.
In recent years, a-diimines 1 (1,4-diazabutadienes) have
received considerable attention due to their widespread applica-
tions as catalyst ligands (Scheme 1). The diimine complexes1
have been utilized as catalysts for a wide repertoire of reactions,
such as alkene polymerization,2a,b alkene–CO copolymer-
ization,2c aziridination,2d allylic amination,2e Suzuki–Miyaura
cross-coupling,2f alkyne hydrogenation,2g alkyne coupling
reactions2h,i and many others.
Other examples of the formation of diimines 2 are given in
Table 1. The imidoyl iodides 3 possessing either electron-
donating (entries 2, 3, 6, 7 and 9) or electron-withdrawing
substituents (entry 5)9 on the N-aryl groups provided 2 in good
yields. For the practical use of a-diimines as a catalyst ligand,
fine tuning of the electronic and steric properties of the N-
substituent of imine moieties plays a significant role in the
selectivity of the catalytic reactions; the selectivities in the
catalytic reactions increase occasionally in accord with increas-
ing steric demand of the N-substituents of the diimine ligands.
The present homocoupling procedure worked well for the
preparation of the fluorinated a-diimines with bulky N-
substituents. The imidoyl iodides 3e–g endowed with the bulky
Scheme 1
Fluorinated a-diimines 2 are considered to be a new class of
catalyst ligands which would dramatically change the catalytic
activities due to the nature of fluorine atom(s). Despite their
potential utility as catalyst ligands,3 to our knowledge, there has
been only one report on the synthesis of fluorinated a-diimines
which possess fluorine atoms in the main chain. Diels et al.
reported a new synthesis of 2,3-bis(trifluoromethyl)- and
2,3-bis(pentafluorophenyl)-1,4-diazabutadienes via the use of
the corresponding fluorinated diketones as a starting material.4
Furthermore, they demonstrated that fluorinated a-diimines
could act as a strong p-acceptor ligand due to their energetically
low-lying LUMO.
We envisioned that the dimerization reaction of fluorinated
imino compounds might be a general reaction leading to the
fluorinated a-diimines. Herein, we report a new access to the
fluorinated diimines 2 which involves palladium(0)-catalyzed
reductive dimerization of the imidoyl iodides 3 in the presence
of carbon monoxide (eqn. 1).
aromatic groups (Ar
=
2,6-Me2C6H3, 2,6-iPr2C6H3 and
2-naphthyl) on the nitrogen afforded the corresponding dii-
mines 2e–g (entries 6–8). Furthermore, the imidoyl iodide 3h
bearing a long perfluoroalkyl chain underwent the reductive
homocoupling to give the symmetrical bis(perfluoroalkyl)dii-
mine 2h in 55% yield (entry 9).
One of the well-established methods for the synthesis of a-
diimines is the condensation reaction of a-diketones with
primary amines. Compared with available methods, the present
methodology merits attention due to the simplicity of the
experimental procedure: (i) the use of only a catalytic amount of
Table 1
(1)
In general, Pd-catalyzed homocoupling reactions of organic
halides require electrochemical reduction or chemical reduction
via the use of Zn, trialkylamines, triphenylphosphine, 2-propa-
nol, hydroquinone, or tetrakis(dimethylamino)ethylene
(TDAE), which can regenerate the catalytically active Pd(0)
species.5–7 Contrary to these extensive studies of aryl–aryl5,6
and vinyl–vinyl7 homocoupling reactions, very little attention
has been devoted to the reductive acyl–acyl and imidoyl–
imidoyl homocoupling reactions, which could be catalytic and
straightforward entries to the a-diketones and a-diimines.
After several attempts under ordinary homocoupling condi-
tions, carbon monoxide was found to be an effective co-
Entrya
Rf
Ar
Diimine 2
Yieldb
1c
2
3
4
5
6
7
8
9
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
C3F7
4-MeOC6H4
4-MeOC6H4
4-MeC6H4
Ph
4-ClC6H4
2,6-Me2C6H3
2,6-(iPr)2C6H3
2-Np
2a
2a
2b
2c
2d
2e
2f
0%
64%
70%
71%
62%
82%
34%
44%
55%
2g
2h
4-MeOC6H4
a Each reaction was carried out in the presence of
5
mol% of
† Electronic supplementary information (ESI) available: synthesis of the
fluorinated diimine 2a. Characterisation details for compounds 2a–2i. See
http://www.rsc.org/suppdata/cc/b3/b303040g/
Pd2(dba)3·CHCl3 (Pd: 0.10 equiv.). b Isolated yield. c The reaction was
carried out under an argon atmosphere (in the absence of CO).
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CHEM. COMMUN., 2003, 1752–1753
This journal is © The Royal Society of Chemistry 2003

wrapped with aluminium foil to minimize exposure to the light. The
reaction mixture was stirred at 70 °C for 7 h. The reaction was monitored by
TLC. When the imidoyl iodide 3a was consumed, the resulting suspension
was filtered through a short celite column (AcOEt). After evaporation of the
solvent, the residue was purified by silica gel column chromatography
(hexane : ether (9 : 1) elute) to give a yellow solid of 2a (0.064 g, 0.16 mmol,
64%): mp 121 °C; IR(KBr) 1598, 1506 cm21; 1H NMR (CDCl3, 200 MHz)
d 6.78 (s, 8 H), 3.80 (s, 6 H); 19F NMR (CDCl3, 188 MHz, C6F6 as an
internal standard) d 93.9 (s, 6 F); MS m/z 404 (M+ 8), 389 (14), 202 (100),
107 (43), 77 (73). Anal. Calcd for C18H14F6N2O2: C, 53.47; H, 3.49; N,
6.93. Found: C, 53.28; H, 3.34; N, 7.23%.
1 For a review of metal-diimine complexes, see: G. van Koten and K.
Vrieze, Advances in Organometallic Chemistry, Academic Press, New
York, 1982, vol. 21, pp. 151–239.
Scheme 2
2 (a) S. D. Ittel, L. K. Johnson and M. Brookhart, Chem. Rev., 2000, 100,
1169; (b) D. J. Tempel, L. K. Johnson, R. L. Huff, P. S. White and M.
Brookhart, J. Am. Chem. Soc., 2000, 122, 6686; (c) J. H. Groen, J. G. P.
Delis, P. W. N. M. van Leeuwen and K. Vrieze, Organometallics, 1997,
16, 68; (d) D. B. Llewellyn, D. Adamson and B. A. Arndtsen, Org. Lett.,
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Chem. Soc., 1996, 118, 11964; (f) G. A. Grasa, A. C. Hillier and S. P.
Nolan, Org. Lett., 2001, 3, 1077; (g) M. W. van Laren and C. J. Elsevier,
Angew. Chem., Int. Ed., 1999, 38, 3715; (h) R. van Belzen, H. Hoffmann
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5 A review on Pd-catalyzed homocoupling: M. Kotora and T. Takahashi,
Handbook of Organopalladium Chemistry for Organic Synthesis, Wiley-
Interscience, New York, 2002, vol. 1, pp. 973–993.
6 Recent reports on Pd-catalyzed reductive homocoupling: (a) A. Jutand
and A. Mosleh, J. Org. Chem., 1997, 62, 261; (b) V. Penalva, J. Hassan,
L. Lavenot, C. Gozzi and M. Lemaire, Tetrahedron Lett., 1998, 39, 2559;
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1999, 1, 1205; (e) D. Albanese, D. Landini, M. Penso and S. Petricci,
Synlett, 1999, 199; (f) S. Venkatraman and C. J. Li, Org. Lett., 1999, 1,
1133; (g) S. Mukhopadhyay, G. Rothenberg, D. Gitis and Y. Sasson, Org.
Lett., 2000, 2, 211; (h) M. Kuroboshi, Y. Waki and H. Tanaka, Synlett,
2002, 637.
7 R. Grigg, P. Stevenson and T. Worakun, Tetrahedron, 1988, 44, 2049.
8 (a) K. Uneyama, J. Fluorine Chem., 1999, 97, 11; (b) K. Tamura, H.
Mizukami, K. Maeda, H. Watanabe and K. Uneyama, J. Org. Chem.,
1993, 58, 32; (c) H. Amii, Y. Kishikawa, K. Kageyama and K. Uneyama,
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a Pd(0) complex allows selective formation of the fluorinated a-
diimines 2; (ii) carbon monoxide as a reducing agent is
inexpensive and easy to remove. Moreover, the starting
materials 3 are more readily available than fluoroalkyl a-
diketones and easily prepared in good yields from commercially
available fluorinated carboxylic acids and primary amines.8b
Next, a further application of this method to the synthesis of
the chiral diimine ligands was explored. The imidoyl iodide 3i
possessing a chiral moiety on the nitrogen atom, which was
prepared from trifluoroacetic acid (TFA) and (S)-phenyl-
ethylamine, underwent reductive homocoupling to afford the
corresponding diimine 2i without any loss of optical purity
(Scheme 2).
In conclusion, we have disclosed a new entry to the synthesis
of the fluorinated a-diimines. Also for the reductive homocou-
pling of imidoyl iodides, we have developed a new catalyst
system Pd(0)–CO, which would possess a broad applicability in
other reductive couplings of organic halides. Further studies are
underway to explore the synthetic applications of the fluori-
nated a-diimines as a ligand to a wide variety of catalytic
transformations including catalytic enantioselective reactions.
This work has been supported by the Sumitomo Foundation
and the Ministry of Education, Culture, Sports, Science and
Technology of Japan (Grant-in-Aid for Young Scientists (B),
No. 14750684 and Grant-in-Aid for Scientific Research on
Priority Areas (“Reaction Control of Dynamic Complexes”),
No. 15036249. We also thank the SC-NMR laboratory of
Okayama University for 19F NMR analysis and the Venture
Business Laboratory of Graduate School of Okayama Uni-
versity for X-ray crystallographic analysis.
9 Crystal data for 2d (Ar = 4-ClC6H4 ): C16H8Cl2F6N2, M = 413.15,
monoclinic, a = 7.2370(8), b = 9.150(2), c = 13.068(2) Å, b =
100.907(9)°, V = 849.7(2) Å3, T = 288 K, space group P2/n (no. 13), Z
= 2, D = 1.615 g cm23. The structure was solved by direct methods (SIR
97), yielding R = 0.0702, Rw = 0.1371 for 1371 independent reflections
with F2 > 5s(F2). CCDC 206534. See http://www.rsc.org/suppdata/cc/
b3/b303040g/ for crystallographic data in .cif or other electronic
format.
Notes and references
‡
A typical procedure for the Pd-catalyzed homocoupling of 3 is as
follows: a two-necked flask attached to a CO (1 atm) balloon was charged
with Pd2(dba)3·CHCl3 (0.025 g, 0.025 mmol) and K2CO3 (0.138 g, 1 mmol),
and toluene (2 mL). Then 0.165 g (0.50 mmol) of trifluoroacetimidoyl
iodide 3a in 2 mL of toluene was added to the catalyst mixture.
Subsequently, 0.05 mL of DMF was added. The reaction vessel was
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